(3) Formalin-Ethyl Acetate (Ether) Concentration. This procedure is used for
the routine recovery of protozoan cysts, helminth larvae, and ova (including operculate
and Schistosoma eggs). The eggs of Hymenolepis nana and the cysts of Giardia lamblia
and Iodamoeba butschlii do not show a good recovery ratio. Recently this procedure has
been made comparatively safe with the replacement of ethyl acetate for ether. However,
ethyl acetate is also a flammable and explosive reagent. Therefore, caution must be
exercised when using this reagent.
(a) Reagents.
1 Physiological saline.
2 Neutral buffered 10
Formaldehyde (USP) ................................... 100.0 ml
Distilled water .............................................. 900.0 ml
Sodium phosphate, monobasic.................... 4.0 gm
Sodium phosphate dibasic ........................... 6.5 gm
3 Ethyl acetate.
(b) Procedure.
STEP 1: Mix a portion of stool about the size of a walnut with about 25 milliliters of
physiological saline (0.85 percent). The ratio of stool to saline should be
controlled to yield approximately one milliliter of sediment.
NOTE:
Tap water may be substituted for saline to destroy Blastocystis hominis.
STEP 2: Strain about 15 milliliters of the suspension through wet gauze held by a small
funnel into a 15 milliliter centrifuge tube.
STEP 3: Centrifuge at 2,500 rpm for two minutes and decant the supernatant. There
should be from one to two milliliters of sediment. If the sediment is too much,
resuspend and discard until the desired amount of sediment is acquired. On
the other hand, if the amount of sediment is not enough, strain more
suspended specimen.
STEP 4: Resuspend in saline or water, centrifuge, and decant. This step may be
repeated until the supernatant is partially clear.
STEP 5: After the last decanting, resuspend the sediment in 10 milliliters of neutral
buffered formalin, mix, and let stand for 15 minutes.
MD0841
2-47